The present invention addresses these and other drawbacks of conventional technology by providing a small planing watercraft with a high-RPM, high-output engine which can make a smooth transition from non-planing to planing movement. The watercraft includes a hull or shell, a propulsion unit arranged in the hull, and an engine arranged in an engine compartment in the hull for directly driving the propulsion unit. The engine includes an exhaust passage, at least one air intake valve, an air intake valve camshaft for opening and closing the air intake valve, and valve timing control means for advancing the normal closure of the air intake valve when the watercraft and engine are operating below a predetermined speed or RPM at which the watercraft transitions from non-planing motion to planing motion. The predetermined speed of the watercraft is directly related to a predetermined engine RPM.
The valve timing control means may include a toothed intake camshaft drive pulley mounted on the end of the intake valve camshaft and operatively connected (e.g., by a toothed belt) with the crankshaft for rotation therewith, and means for selectively rotating the pulley relative to the intake camshaft for advancing the closure of the air intake valve when the engine is operating below the predetermined speed. More particularly, the selective rotating means may include an inner shaft fixed on the end of the intake valve camshaft and having helical splines arranged on its outer surface. An annular sliding piston is slidably arranged around the inner shaft with helical splines on its inner surface for engaging the splines on outer surface of the inner shaft. The piston also has oppositely twisted helical splines on its outer surface for engaging similar splines inside an interior cylindrical opening of the pulley. All of the splines are arranged so as to rotate the pulley relative to the camshaft in response to axial translation of the annular piston.
The engine may also include a long air intake passage for low RPM operation and a short air intake passage for high-RPM operation, with an air intake control valve provided in the short air intake passage that includes means for closing the air intake control valve when the watercraft is operating below a predetermined speed. The engine may include an exhaust passage having an exhaust control valve for selectively constricting the exhaust passage by partially closing the exhaust control valve when the watercraft is operating below the predetermined speed.